Refining suitable petroleum crude oils to obtain a variety of lubricating oils which function effectively in diverse environments has become a highly developed and complex art. Although the broad principles involved in refining are qualitatively understood, there are quantitative uncertainties which require considerable resort to empiricism in practical refining. Underlying these quantitative uncertainties is the complexity of the molecular constitution of the precursor crude fractions. Because these crude fractions boil above about 550.degree. F., the molecular weight of the constituents is high and these constituents display almost all conceivable structures and structure types including molecules that contain, in addition to carbon and hydrogen, metals, nitrogen, oxygen and sulfur, collectively referred to hereinbelow simply as "heteroatoms". This complexity and its consequences are referred to in "Petroleum Refinery Engineering", by W. L. Nelson, McGraw Hill Book Company, Inc., New York, N.Y. 1958 (Fourth Edition), relevant portions of this text being incorporated herein by reference for background.
The basic notion in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant base stock having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant base stock currently consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining, and dewaxing, which basically are physical separation processes in the sense that if all the separated fractions were recombined one would reconstitute the crude oil.
A lubricant base stock (i.e. a refined oil) may be used as such as a lubricant, or it may be blended with another lubricant base stock having somewhat different properties. Or, the base stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, and V.I. improvers. As used herein, the term "stock" regardless whether or not the term is further qualified, will refer only to a hydrocarbon oil without additives. The term "raw stock" will be used herein to refer to an untreated viscous distillate or to the residuum isolated by vacuum distilling an atomospheric tower residuum of a petroleum crude, or its equivalent. The term "solvent-refined stock" or "raffinate" will refer to an oil that has been solvent extracted, for example with furfural. The term "dewaxed stock" will refer to an oil which has been treated by any method to remove or otherwise convert the wax contained therein and thereby reduce its pour point. The term "waxy", as used herein, will refer to an oil of sufficient wax content to result in a pour point greater than +25.degree. F. The term "stock", when unqualified, will be used herein generically to refer to the viscous fraction in any stage of refining, but in all cases free of additives. The term "base stock", or "lube base stock", will refer to an oil refined to a point suitable for some particular end use, such as for preparing automotive oils.
Briefly, for the preparation of lube base stocks, the general practice is to vacuum distill an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks within the boiling range of about 550.degree. to 1050.degree. F. and a vacuum residuum. After preparation, each raw stock is extracted with a solvent, e.g. furfural, phenol or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components. The vacuum residuum usually requires an additional step to remove asphaltic material prior to solvent extraction. The raffinate from solvent refining is then dewaxed by admixing with a solvent such as a blend of methyl ethyl ketone and toluene, for example. The mixture is chilled to induce crystallization of the waxes which are then separated from the dewaxed dissolved raffinate in quantity sufficient to provide the desired pour point for the subsequently recovered dewaxed raffinate.
In general, refineries do not manufacture a single lube base stock but rather process at least one distillate fraction and the vacuum residuum. For example, three distillate fractions differing in boiling range and the residuum may be refined. These four fractions have acquired various names in the refining art, the most volatile distillate fraction often being referred to as the "light neutral" fraction or oil. The other distillates are called "intermediate neutral" and "heavy neutral" oils. The vacuum residuum, after deasphalting, solvent extraction and dewaxing, is commonly referred to as "bright stock".
Viscosity Index (V.I) is a quality parameter of considerable importance for lubricating oils to be used in automotive engines and aircraft engines which are subject to wide variations in temperature. This index is a series of numbers ranging from 0 to 100 which indicate the rate of change of viscosity with temperature. A Viscosity Index of 100 indicates an oil that does not tend to become viscous at low temperature or become thin at high temperatures. Measurement of the Saybolt Universal Viscosity of an oil at 100.degree. and 210.degree. F., and referral to correlations, provides a measure of the V.I. of the oil. For purposes of the present invention, whenever V.I. is referred to it is meant the V.I. as noted in the Viscosity Index tabulations of the ASTM(D567), published by ASTM, 1916 Race St., Philadelphia Pa., or equivalent.
To prepare high V.I. automotive and aircraft oils the refiner usually selects a crude oil relatively rich in paraffinic hydrocarbons, such oils being referred to commonly as "paraffin base" or "mixed base" crudes, since experience has shown that crudes poor in paraffins, such as those commonly termed "naphthene-base" crudes, yield little or no refined stock having a V.I. above about 40. (See Nelson, supra, pages 80-81 for classifications of crude oils). Suitable stocks for high V.I. oils, however, also contain substantial quantities of waxes which result in solvent-refined lubricating oil stocks of high pour point, i.e. a pour point greater than +25.degree. F. Thus, in general, the refining of crude oil to prepare acceptable base stocks ordinarily includes dewaxing to reduce the pour point to a target value less than +25.degree. F.
Raw distillate lubricating oil stocks usually do not have a particularly high V.I. However, solvent-refining, as with furfural, for example, in addition to removing unstable and sludge-forming components from the crude distillate, also removes components which adversely affect the V.I. Thus, a solvent-refined stock prior to dewaxing usually has a V.I. well in excess of specifications. Dewaxing, on the other hand, removes paraffins which have a V.I. of about 200, and thus reduces the V.I. of the waxy stock.
In recent years catalytic techniques have become available for dewaxing petroleum stocks. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal dated Jan. 6, 1975, at pages 69-73. See also U.S. Pat. No. 3,668,113.
In U.S. Pat. No. Re. 28,398 to Chen et al. is described a process for catalytic dewaxing with a catalyst comprising Zeolite ZSM-5. Such processes combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938. U.S. Pat. No. 3,755,138 to Chen et al. describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point. The entire contents of these patents are herein incorporated by reference.
The catalytic dewaxing process described in the foregoing Chen et al., for example, references is normally started at as low a temperature (start of run temperature) as will provide a product meeting the target pour point specification. The catalyst activity usually declines relatively fast during the first days on stream, as made evident by an increase in pour point of the dewaxed raffinate, and the decline in activity is compensated by gradually increasing the dewaxing temperature. After some days, a second period sets in, sometimes referred to as the "line-out period", during which catalyst activity declines very slowly and requires a compensatory small increase in temperature, for example, 1.degree. F./day or less. When the temperature reaches a predetermined level (end of run temperature) above which the dewaxed product lacks stability prerequisites, the run is terminated to allow catalyst regeneration. The term "aging" will be used herein to refer to the catalyst deactivation which occurs either initially or during line-out operation. The term "catalyst cycle life" or "cycle life" as used herein means the length of time that elapses between the start of run temperature and the end of run temperature.
Catalyst aging in dewaxing is often ascribed to impurities in the waxy raffinate feed. While not wishing to be limited by theory, it seems reasonable to assume that certain of the organic compounds that have metals, nitrogen, oxygen or sulfur in their structure interact with and deactivate the acidic sites of the catalyst; and, that these and/or other impurities function as coke precursors which interact with the catalyst matrix to form deposits of carbonaceous catalyst residues. The CCR (Conradson Carbon Residue) of a stock is believed to be indicative of the propensity of a stock to form such catalyst residues. Also contributing to impurities in the waxy raffinate are the limitations of the solvent-refining process itself, which, due to the high viscosity and high molecular weight of lube stocks, diminish the efficiency of the solvent extraction and deasphalting steps.
A number of waxy raffinates from different crudes that can be dewaxed by the solvent process are not now efficiently dewaxed catalytically because of a prohibitively short cycle life. Also, waxy raffinates that are now or may be catalytically dewaxed would benefit from a slower rate of deactivation and extended cycle life.